Field
Inductive displacement sensors and methods of using them may be useful in a variety of contexts. For example, systems for precisely measuring linear or angular motion may use inductive displacement sensors to measure changes in position.
Description of the Related Art
In certain conventional inductive sensors, a primary coil and two secondary coils are provided. In such sensors, the two secondary coils are designed such that the magnitudes of the received signals vary inversely, with respect to one another, with mechanical motion. The early versions of such sensors are known as linear variable differential transformers (LVDT).
The LVDT is an inductive transducer that converts a linear displacement or angular motion relative to a mechanical reference (or zero) into a proportional electrical signal containing phase information (for direction) and amplitude information (for distance). FIG. 1 illustrates typical LVDT configurations.
As shown in FIG. 1, the LVDT consists of a primary coil of wire wound over the whole length of a non-ferromagnetic bore liner or spool tube or over the whole length of a cylindrical, non-conductive material such as a plastic or a ceramic material, in coil form or bobbin. Two secondary coils (Secondary a, Secondary b) are wound around the primary coil. These two secondary coils are typically connected in opposite series or in a differential connection. The secondary coils are symmetrical with respect to one another, such that they effectively form a sine-cosine relationship.
More recent inductive sensors replace the transformer found in a LVDT with coils printed on a PCB. This substitution results in similar magnetic coupling effects with a substantial reduction in size and cost of the sensor. As with LVDT, the primary coil is driven with a sine wave carrier. A coupler, typically another PCB configured with PCB traces that form a shorted coil, is used to couple the field created by the primary coil to the secondary coils such that the amplitude of the received signals varies with mechanical movement of the coupler.
The signals received on the secondary coils provide feedback to the system resulting in automatic gain control (AGC.) This feedback relies on a sine/cosine relationship between the two received signals, which supports the equation: A2+B2=K, wherein K is a constant. The difference between the calculated K and the ideal value of K is utilized as the feedback signal for the system. The feedback can be utilized to vary the magnitude of the sine wave driven into the primary coil and/or to vary the gain of an amplifier in the signal conditioning path of the system. Because the feedback applies to both signals equally, the ratiometric relationship of the two signals remains intact, and each output continues to proportionally reflect the input.
The signals received on the secondary coils are also utilized to measure mechanical motion. In the case of conventional dual coil inductive sensors, the secondary coils have a sine/cosine relationship which requires signal processing in order to produce a tangent or cotangent result. This tangent/cotangent result must then be processed further, by a look-up table or other means, to create a linear or other output indicative of motion of the coupler. Such signal processing requires a system which includes a processor, memory, and signal processing algorithms.